1. Technical Field
The present invention relates to methods of operating fuel cell stacks, as well as corresponding systems.
2. Description of the Related Art
Electrochemical fuel cells convert reactants to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte, such as an ion-exchange membrane, interposed between two electrodes, namely an anode and a cathode, to form an electrode assembly. The electrode assembly is typically interposed between two electrically conductive flow field plates or separators that act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such separators typically contain flow fields to supply reactants, such as fuel and oxidant, to the anode and the cathode, respectively, and to remove excess reactants and reaction products, such as water formed during fuel cell operation. Typically, a number of fuel cells are connected in series to form a fuel cell stack.
Fuel cell stacks may be operated in many different ways, including dead-ending the anode and recirculating the fuel. In such modes of operation, nitrogen from the air in the cathode typically crosses over through the ion-exchange membrane to the anode due to a concentration gradient. As fuel is consumed in the fuel cell, the concentration of nitrogen in the anode increases. This accumulating of nitrogen negatively impacts the performance of the fuel cell stack. In addition to nitrogen, minute amounts of impurities from the fuel source can also build up in the anode as fuel is consumed and negatively impact operation of the fuel cell.
To remove excess nitrogen and other impurities from the anode, fuel cell systems often contain a purge assembly, such as a purge valve, downstream of the fuel cell stack to periodically purge the fuel exhaust. A purge valve is typically a solenoid purge valve, such as a two-way (open-close) solenoid valve, or a pulse-width modulated (PWM) valve. However, if the purging conditions are not properly defined, an undesired amount of hydrogen may be purged in the fuel exhaust, thereby decreasing fuel efficiency and possibly creating a flammable environment.
To avoid these problems, gas sensors have been used extensively in fuel cell systems for determining the hydrogen concentration of the fuel exhaust to ensure that a minimal amount of hydrogen is purged from the fuel cell stack. For example, U.S. Patent Application Publication No. US2002/0110713 discloses a gas sensor in the interior fluid passages of a fuel cell assembly, or within fluid passages employed to transport reactant fluid streams to and/or from the fuel cell(s).
Hydrogen sensors are typically expensive and may be unreliable. For example, PCT Publication No. WO2008/018243 discloses a hydrogen gas concentration sensor which comprises a base and hydrogen-detecting films. These hydrogen-detecting films have a thin film layer and a catalyst layer which, upon contact with hydrogen gas, hydrogenates the thin film layer to reversibly change its electrical resistance. These thin film layers have a higher sensitivity when the hydrogen gas concentration is low, and a wider determination range when the hydrogen gas concentration is high. Such hydrogen gas concentration sensors, however, are complicated and expensive to make, and are subject to degradation over the lifetime of the fuel cell stack.
Other techniques to measure the concentration of hydrogen involve measuring the pressures upstream and downstream of the fuel cell stack, and using the pressure difference to determine the hydrogen concentration. However, water droplets in the flow field channels of the fuel cell often produce inaccurate downstream pressure measurements, and thus an inaccurate estimate of the hydrogen concentration.
As a result, there remains a need for improved methods of determining the hydrogen concentration, particularly in the fuel exhaust stream of a fuel cell stack. The present invention addresses this need and provides other related advantages.